1. Field of the Invention
This invention relates to optical test apparatus, and in particular to apparatus for detecting and monitoring losses and/or faults in optical fibres.
2. Related Art
It is well known that faults in optical fibres can be located by an optical time domain reflectometer (OTDR). An OTDR launches a pulse of light into a fibre, and backscattered light is monitored for abrupt changes indicative of a loss or fault. The distance of the loss or fault from the launch end of the fibre can be determined from the time interval between launch and return of the backscattered peak. Once a period of time sufficient to receive all detectable backscattered light has passed, a further pulse may be launched into the fibre. The pulse width may be varied for different dynamic range or resolution requirements. Thus, for a given amplitude, an increase in the pulse width enables a greater length of fibre to be monitored, that is to say it increases the dynamic range of the OTDR. The dynamic range of an OTDR is the loss after which an event, backscatter or reflection can still be detected.
The OTDR is, therefore, an extremely useful item optical test equipment; since, from connection to a single end of an optical fibre network, the location of losses and reflections can be determined, and their amplitude measured, to a high degree of accuracy. For loss measurements, both point losses and end-to-end fibre or network losses can be measured. In duplex networks, the amplitude of any reflections is important, since these may cause crosstalk. Using wavelength division multiplexing (WDM) techniques, it is also possible to take these measurements at a particular wavelength whilst the network is carrying data at another wavelength.
Until recently, high dynamic range OTDRs have primarily been required for use on long (&gt;75 km) submarine systems. In order to achieve the dynamic range required, the width of the output pulse is increased so that more power is launched into the fibre. This has the effect of reducing the spatial resolution (that is to say the minimum distance between which events can be distinguished). Typically, a 10 .mu.s pulse would result in a splice loss appearing 2 km in length. For long-haul systems, where events are few and far between, this resolution is sufficient.
With the advent of Passive Optical Networks (PONs) which are short (&lt;10 km) in length, but high in loss (25 dB), a requirement for a high resolution (&lt;10 m), high dynamic range (&gt;25 dB backscatter), OTDR is apparent. Currently available OTDRs which have the necessary spatial resolution fall short on dynamic range. Thus the Ando 7110C, with a 50 ns pulse width (10 m resolution), has a backscatter dynamic range of 15 dB; and the Anritsu 910C, with a 100 ns pulse width (20 m resolution), has a backscatter dynamic range of 18 dB. It should be noted that these dynamic range figures are obtained by measuring the loss of the fibre which can be seen from the point nearest to the OTDR to the point where the peak of the noise floor is reached after about 10 minutes of trace averaging. In practice, depending on the measurement to be taken, the usable dynamic range may be up to 3 dB less.